This weapon has been blessed, perhaps by the deity to which its user swears their oath.
Book of Exalted Deeds
James Wyatt, Christopher Perkins, Darrin Drader
This replicates the effect of the bless weapon spell, but it is permanent and always in effect. The weapon becomes good, which means it can bypass the damage reduction of certain creatures, particularly evil outsiders. Furthermore, all critical hit rolls against evil foes are automatically successful, so every threat is a critical hit. This last effect does not apply to any weapon that already has a magical effect related to critical hits, such as a keen weapon or vorpal weapon.